HEREDITY OF DISEASES. 23 



themselves, are not beneficial. It may, in fact, be 

 said that any peculiarity in the functional activity of 

 an organ, if long continued, may result in a habit of 

 the system which the offspring will in all probability 

 inherit. 



Hereditary disease may make its appearance at the 

 time of birth, when it is said to be congenital, or a 

 considerable length of time may elapse before any 

 indications of its presence are observed. In the latter 

 case & predisposition or tendency to the disease is said 

 to be inherited, which often requires some external 

 exciting cause for its full development. 1 



There are certain diseases that are transmitted 

 with greater uniformity than others ; yet a predispo- 

 sition to almost every known form of disease is likely 

 to become hereditary, even if the influence that deter- 

 mines its transmission is not sufficiently intense to 

 render it congenital. It is not my purpose to describe 

 or even enumerate all the diseases that are known to 

 be hereditary, but to notice only those that illustrate 

 the laws of hereditary transmission, or that, from 

 their frequent occurrence, are of particular interest 

 to breeders of domestic animals. Under the general 

 term " scrofula," a great variety of disorders are in- 

 cluded, all of which are characterized by a perversion 

 of the nutritive functions, and the formation of pe- 

 culiar tumors, called tubercles, in the various organs 

 of the body. The most common forms of scrofulous 



1 Adams on " Hereditary Diseases," p. 19; Williams's "Principles 

 of Medicine," p. 47; Paget's "Surgical Pathology," p. 514; "Cyclo- 

 paedia of Anatomy and Physiology," vol. ii., p. 471 ; Aitken's " Science 

 and Practice of Medicine," vol. ii., p. 35. 



